Although the archway is the sensitive part of the metal detector, all detectors can be subject to "noise" from the outside environment. Typical sources of such noise are electrical interference from large motors, fluorescent lamps, copy machines, and radio transmissions. Nuisance alarms can also be caused by people moving large metal objects outside, but near the archway. It is also possible that an operator can cause nuisance alarms by leaning on the archway, causing mechanical vibration or by moving too close with objects such as radios, badges, weapons, and other security hardware.
Occasionally, outside interferences will trigger the alarm when someone is in the archway and is not carrying a detectable metal object. Thoroughness requires that if the alarm sounds, the individual must be examined for metallic objects with a handheld metal detector and/or by passing through the archway again, even if a nuisance alarm is suspected.
It would not be unusual for a person carrying a weapon to try to disguise the fact by bumping the archway as he moves through, and then try to convince the operator that the vibration was the source of the alarm. He might also try to convince the operator that the response was from some other object he was carrying and that no further investigation is needed. Always investigate all alarms thoroughly. Investigate the unusual.